This invention relates to promoting the dewatering of sludges by the addition of a flocculating system to the sludge prior to the dewatering.
It is standard practice to flocculate a suspension of suspended solids by adding a polymeric flocculating agent to the suspension.
Different suspensions, and different dewatering processes, require different flocculating agents in order to obtain optimum results. Further, the optimum results vary according to the nature of the suspension being treated.
For instance, when the suspension has a relatively low solids content typically below 1% and usually below 0.5% and often below 0.2%, the optimum result is generally indicated by achieving the highest possible clarity of the supernatant or filtrate. When the suspension is a paper making cellulosic thin stock, the optimum result is generally indicated by optimum formation of the paper sheet, generally accompanied by optimum retention of suspended solids in the sheet. When the suspension is a waste sludge, optimum results are generally indicated by a good combination of dewatering rate, clarity and solids content of the dewatered product. For instance, it may be unacceptable to obtain a high solids cake in a very fast dewatering process if the resultant supernatant or filtrate clarity is poor, but it may be uneconomic to obtain a high solids cake and high clarity if the rate of dewatering is too slow.
The suspension solids in a low solids liquor which is to be clarified are usually different from those in a sludge. For instance a deinking was liquor may contain inks, fillers and deinking chemicals, and these are separated during clarification. A primary sludge which incorporates those separated solids is much more complex and also contains large amounts of different materials having very different properties, such as rejects from screening, cleaning and flotation deinking and other paper mill processes
A polymer that may be useful when dewatering is by one process, may not be useful when it is by another. For instance different polymers may be required in processes where shear or pressure is applied to the flocculated material (for instance in a filter press or centrifuge) from the polymer where no such shear or pressure is applied (for instance in sedimentation processes).
Different polymers are also required according to differences in the suspended solids and in the dissolved phase. For instance suspensions of inorganic suspended solids are often best treated using anionic polymers while suspensions of organic suspended solids are often best treated using cationic polymers.
The molecular weight of the polymer also influences performance and although optimum flocculation in some processes requires the highest possible molecular weight, in other processes lower molecular weight gives better results. Even within any particular type of suspension, e.g. a cellulosic papermaking thin stock, different types of thin stocks require different types of polymers for optimum results.
There is therefore a very wide range of polymeric flocculants available for consideration for use in dewatering processes. They are selected from nonionic polymers, anionic polymers and a wide range of cationic polymers. Many are substantially water soluble polymers formed from ethylenically unsaturated acrylic or other monomer or monomer blends in various portions and made to low, medium, high or very high molecular weight. Amongst other polymers used for promoting dewatering there are polyamines, polyalkylene oxides, polyethylene imines, phenolic resins and dicyandiamide polymers.
There are numerous examples of processes in which water soluble acrylic polymers are used for flocculation, for instance EP-A-641,293 for sludges from paper-making waste water and EP-A-235,893 for paper making. Examples of processes using phenol formaldehyde and polyethylene oxide include U.S. Pat. No. 5,354,479 and CA 1,004,782 for treating paper-making waste waters, and WO95/21296 for paper making. An example of a process where it is proposed to use either a substantially non-ionic acrylic polymer or PEO for paper making is EP-A-017353.
The polymers usually used for promoting the dewatering of sludges are usually relatively high molecular weight, water soluble, cationic polymers such as polymers of 20 to 95% by weight acrylamide and 5 to 80% by weight of an acid addition or quaternary salt of a dialkylaminoalky (meth)-acrylate or acrylamide.
These polymeric acrylic flocculants give good results in many sludge dewatering processes. However there are some particularly difficult sludges where it is necessary to use unacceptably large amounts of these acrylic polymers to obtain satisfactory results, or where satisfactory results cannot economically be obtained using such polymers. The difficult sludges which suffer from this disadvantage tend to be those which include secondary sludges (especially when some or all of the sludges are derived from waste associated with paper making) and the sludges which include significant amounts of deinking wastes, recycling wastes and mechanical pulping wastes.
Primary sludges are sludges obtained by clarifying and sedimenting an aqueous liquor which may be, for instance, municipal sewage or effluent from a paper mill, pulp mill or deinking plant. The separation of the primary sludge from the initial liquor results in the formation of a supernatant or filtrate and this is then usually subjected to biological treatment to form what is usually referred to as a secondary sludge. It is usually much more difficult to dewater the secondary sludge than the primary sludge and so it is common practice to mix the primary and secondary sludges and then dewater the mixed sludge. This is true for municipal sewage treatment and also for industrial waste treatment, in particular the treatment of paper mill, pulp mill and deinking plant effluent.
As an indication of the increased difficulty of dewatering secondary sludge, it can be mentioned that a typical primary sludge usually requires 0.1 to 1.5 kg of an optimum acrylic polymeric flocculant per tonne of dry sludge solids whilst a primary sludge from a deinking, recycling or mechanical pulping plant may require 2 to 4 kg of optimum acrylic polymeric flocculant per tonne of dry sludge solids, and a secondary sludge usually requires 5 to 10 kg of optimum acrylic polymeric flocculant per tonne of dry sludge solids.
Sludges which contain secondary sludge, especially from paper making, pulping and deinking effluents, often contain a large proportion of organic residues which tend to interfere with the flocculation and dewatering mechanisms. As a result, the rate of dewatering and/or the clarity of the filtrate or supernatant and/or the dryness of the cake solids tend to be unsatisfactory even when relatively large amounts of polymeric flocculant are used on the sludge. The polymers which have been accepted as usually being the most efficient for such sludges are usually high molecular cationic acrylic polymers, sometimes in combination with low molecular weight cationic polymers such as polydimethyl diallyl ammonium chloride.
It would be desirable to be able to improve the dewatering of difficult sludges such as those so as to obtain an improved combination of dewatering rate and/or filtrate or supernatant clarity and/or cake solids, especially when the sludges originate (at least in part) from a paper mill, pulp mill or deinking plant.